ABSTRACT: Assessments of the biodiversity and structure of coral reef fish communities are often plagued by inadequate sampling and biases inherent in commonly used Underwater Visual Census (UVC) methods. Of these biases, heterogeneity in the detectability of reef-fish species is often ignored, even though it may have substantial effects on biodiversity estimates. Using highly replicated UVC sampling of all fish species at 4 sites in Tanzania, East Africa, we show that detectability varies greatly across species and is affected by traits such as body size and schooling behaviour, and that detectability of reef fishes can be readily accounted for by the application of Capture-Mark- Recapture (CMR) models. Based on our results, we recommend that approximately 24 point counts are necessary to assess full reef-fish species richness at sites in the Western Indian Ocean.